The invention relates to a programmable multifunction speedometer having particular suitability for use in motor vehicles.
In an electronic programmable speedometer, the speedometer receives a signal from a sensor, i.e., a series of electrical pulses which are proportional to the speed of the vehicle. By measuring the period or frequency of the signal, vehicle speed is determined. Although a relatively simple calculation, the number of pulses per unit distance must be precisely calibrated to the mechanical characteristics of the vehicle, in particular, variables such as tire size, axle gear ratio, and the number of intermediate gear teeth.
Calibration of conventional speedometers is done using linear equations which factor in each of the vehicle's mechanical characteristics. By computing the solution to the equation, a number is obtained which, when looked up in a table, provides appropriate programming for the vehicle. Programming is then accomplished by selected positioning of small switches located on the back of the speedometer. The size and location of these switches, however, has made them difficult to access and read. Also, the number of programming possibilities is limited by the number of switches which may be practically located on the device.
Conventional programming must therefore be done prior to speedometer installation. When vehicle characteristics, e.g., the tire size or gear ratio, are modified subsequently, the speedometer must be physically removed from the vehicle and reprogrammed, or replaced with a new speedometer with appropriate calibration.